1. Field of the Invention
The present invention relates to a transmitter and a mobile telephone using the apparatus which are capable of switching a plurality of frequencies.
2. Description of the Related Art
A transmitter for switching a plurality of frequency bands has been heretofore considered to have a constitution as shown in a block diagram of FIG. 6. In this drawing, there is illustrated a transmission of two frequency bands. Such a transmitter is considered to be particularly used for a mobile phone or the like.
As shown in FIG. 6, the conventional transmitter comprises a modularized power amplifying section 551; an antenna 510; an antenna switch section 552, in the form of IC, for switching the antenna 510; and an external auxiliary matching section 553.
The constitution of the modularized power amplifying section 551 is as follows.
That is, an input wide band matching circuit 501 is connected to an input port 511 to which signals of the first and second frequency bands are inputted. A power amplifying transistor 502 is connected to an output port of the input wide band matching circuit 501. An output main matching circuit 503 is connected to the power amplifying transistor 502.
The antenna switch section 552 comprises transistors 504, 505 connected in series between the output port of the output main matching circuit 503 and the antenna 510; transistors 506, 507 connected in series between the output port of the output main matching circuit 503 and ground; and control ports 513, 514 for them.
The external auxiliary matching section 553 comprises an external capacitance 509 that is for an auxiliary matching circuit; and an external switch 508 for connecting/disconnecting the capacitance 509 and the output main matching circuit 503.
In the above constitution, in the first frequency band, the power amplifying transistor 502 is matched enough for the high efficiency linear amplification by the input wide band matching circuit 501 and the output main matching circuit 503. In the second frequency band, the input side is matched by the input wide band matching circuit 501. However, the output side is not matched enough for the high efficiency linear amplification by the output main matching circuit 503 alone. Thus, the external switch 508 is turned on so as to accomplish the high efficiency linear amplification by the use of the capacitance 509. This high efficiency linear power amplification brings about an effect of increasing a battery longevity of the mobile phone.
When these amplified signals are transmitted, gate voltages applied to the control ports 513, 514 are controlled. Thereby, the in-series transistors 504, 505 operated as the antenna switch are turned on, while the similarly in-series transistors 506, 507 are turned off.
On the other hand, when the signals are not transmitted, the transistors 504, 505 are turned off while the transistors 506, 507 are turned on, whereby the output side is grounded.
The two transistors 504, 505 are connected in series for the following reason. That is, when a transmission output is as high as about 1 W, one transistor alone causes a leakage current and thus it is not completely turned off. Therefore, two transistors are connected in series in order to ensure that the transistor is completely turned off. The transistors 506, 507 are connected in series for the same reason.
However, such a conventional transmitter has a problem as described below. That is, this apparatus requires the external switch for switching the matching circuit so that it may comply with a plurality of frequency bands. Due to this, the number of components are increased. Moreover, this external switch circuit increases a loss.